Bacteriophage T4 RNA ligase will be studied by a combined enzymological and genetic approach. To explore the physiological role of the enzyme, the T4 mutants we have isolated will be examined for the defect in their metabolism. We have already shown a defect in phage morphogenesis. We have also shown that T4 RNA ligase promotes joining of duplex DNA at base-paired ends. This reaction will be exploited for constructing chimeric DNA; i.e. DNA from two different sources. Finally, the enzyme will be used to introduce into DNA the sequence of bases that are recognized by specific restriction enzymes.